Fast Signal Recovery for Multi-user MIMO-OFDM Uplink with Frequency Offsets

In the uplink multi-user MIMO-OFDM system, usually there are multiple carrier frequency offsets (CFOs) between the users and the base-station. The CFOs destroy the orthogonality among subcarriers and cause inter-carrier interference (ICI) in the received signals, and therefore make the "per-tone" equalization invalid. Since there are multiple different CFOs, the conventional CFO compensation method for OFDM can no longer be used. In this paper, a method is proposed to incorporate the CFOs into the transmitted symbols and channels, and re-model the MIMO-OFDM with CFOs into a MIMO-OFDM without CFOs. The "per-tone" equalization is used to recover the CFO incorporated transmitted signals. Then the real transmitted signals are obtained by depriving the CFOs from them. Compared with the equalization method for the MIMO-OFDM without CFOs, here the computational complexity only increases by two fast Fourier transforms (FFTs) for each user